Call handoff

ABSTRACT

A system and method for performing call handoff in a mobile communication system. The system is employed on commercial aircraft and enables calls to be transferred between different traffic service channels in order to continue and/or improve communications. In a conservation handoff scenario, user service channels are transferred between partially utilized traffic service channels resulting in unused traffic service channels that are released, thus freeing more traffic service channels for use by other aircraft. In a seizure handoff scenario, signal quality on one traffic service channel has begun to deteriorate prompting a call handoff wherein a call on the deteriorating traffic service channel is transferred to an unused user service channel on another traffic service channel already in use aboard the aircraft, thus allowing users to continue their calls. In a reservation handoff situation the aircraft equipment responds to a deterioration of signal quality and transfers all calls to another traffic service channel on a different radio base station allowing user&#39;s to continue their calls.

This application is a continuation of Ser. No. 08/509,703 filed Jul. 31,1995.

FIELD OF THE INVENTION

The present invention relates generally to multiple communicationdevices sharing a limited amount of available electromagnetic spectrum.More particularly, the present invention relates to more efficient andeffective usage of communication channels associated with telephonicdevices employed on airplanes.

BACKGROUND OF THE INVENTION

The electromagnetic spectrum is a limited and valuable resourceallocated in the United States by the federal government, specificallythe Federal Communications Commission (FCC). The FCC determines whichtypes of applications are permitted to use which parts of theelectromagnetic spectrum. Two radio frequency bands have been allocatedby the FCC for use by airborne telecommunication systems. Communicationswith airborne telephones on commercial aircraft has been allotted bandsfrom 849 to 851 megahertz (MHz) for uplink communications, i.e.transmissions to the airborne telephones, and from 894 to 896 MHz fordownlink communication, i.e., transmissions from airborne telephones.Each band has 2 megahertz (MHz) bandwidth, and the two bands areseparated by 45 MHz. Both the uplink and downlink bandwidths are dividedinto 10 subbands, each 200 kilohertz (KHz) wide. The subbands arefurther divided into 29 traffic service channels (a type ofcommunication channel) and six pilot channels each. Thus a total of 290traffic service channels are available for communication with airbornetelephones. Each traffic service channel has a 6 KHz bandwidth in boththe uplink and downlink frequency allotments.

As shown in FIG. 1 the electromagnetic broadcast frequency spectrum 10allotted for communications with airborne telephones has a low band 12and high band 14. Each band 12,14 has been divided into 10 subbands16,18 of 200 KHz each, numbered from 10 down to 1. Each subband 16,18has been further divided into a set of 6 numbered control channels(pilot channels) 20 and 29 traffic service channels 22. In accordancewith the FCC Memorandum of Opinion and Order, each of the six controlchannels 20 has been given a bandwidth of 3.2 KHz, and each of the 29traffic service channels 22, a bandwidth of 6 KHz. Guard bands of 2.5KHz 24, 2.3 KHz 26 and 1.5 KHz 28 separate traffic service channels 22from pilot channels 20 and from traffic service channels in differentsubbands. These channel assignments allow up to six service providers tooffer nationwide airborne radiotelephone services simultaneously. Eachwill be assigned one of the numbered pilot channels. The pilot channelassigned to a specific service provider will be the same in each subbandin each cell covered by a radio base station. All service providers willhave equal access to the set of traffic service channels used in eachcell. No service provider “owns”: a traffic service channel, but each“owns” one control channel in each subband.

Finally, it is known to divide each traffic service channel into 2 userservice channels. Each user service channel carries the communicationsbetween a phone on an airplane and another phone coupled to a radio basestation. That radio base station must be serving a geographic area inclose enough proximity to the aircraft to allow communication with theairplane.

The United States is blanketed with dozens of radio base stations. Aradio base 5 station is the suite of ground equipment required toprocess air-to-ground and ground-to-air calls. The ground stations arelocated throughout the U.S. as well as Canada and Mexico. Typically, oneradio base station is separated from another by 300 to 500 miles. Eachradio base station is assigned a block of frequencies or subband(s) onwhich calls are processed. Subbands are assigned such that the samesubband is not reused within 550 miles from the radio base station it isassigned to. This arrangement avoids co-channel interference, i.e., thesame channel in use in overlapping cells.

The limited bandwidth allotted to communication with airborne telephonesin combination with the number of available radio base stations servesto constrain the possible number of simultaneous calls, thus limitingthe market for airborne telephonic communications. Therefore it isdesirable to provide a system capable of utilizing the availablespectrum with as high efficiency as possible while providing excellentquality communications to airborne customers.

When an aircraft radio unit on an aircraft acquires a traffic servicechannel both user service channels may or may not be utilized for aperiod of time, but typically one user service channel becomes unusedbefore the other. This is because each user service channel is beingused independently, i.e., any given call utilizing a user servicechannel is usually unrelated to a call utilizing the other user servicechannel on the same traffic service channel. In that case the aircraftwill only be using one user service channel per traffic service channel,even though two user service channels are available per traffic servicechannel. This often happens on multiple traffic service channelsresulting in multiple traffic service channels only being partiallyutilized. Note that present aircraft radio units have only two trafficservice channels each. Unfortunately, partial utilization of multipletraffic service channels by one aircraft will preclude other aircraftfrom acquiring those traffic service channels or using the unused userservice channel. Recall that a total of 290 traffic service channels isall that is presently available for airborne telephones. In presentairborne telephone systems the described management of Traffic servicechannels can result in callers on other aircraft being precluded frommaking calls while unused user service channels exist but areunavailable. Therefore, it would be desirable for traffic servicechannels to be used more efficiently to minimize the number of partiallyutilized traffic service channels in order to increase the number ofpotential simultaneous calls.

Other problems arise with a mobile communications system, especially onewhich is deployed on commercial jet aircraft. For example, aconversation between a passenger on such a plane and someone on theground or in another plane may continue long enough for the plane to flyfrom one cell into another. Note that in this case a cell is defined asthe area wherein a radio base station provides a signal above athreshold necessary to provide quality communications and that cellareas may overlap. When this happens the call is eventually terminatedas an aircraft flies out of an acceptable communications range.Presently, the only way to continue the conversation is for one of theparties to the conversation to redial the other party. It would bedesirable for the mobile communications system to retain the connectionbetween callers even though at least one of the callers is moving fromone cell to the next. Furthermore, it would be desirable to retain theconnection between callers with a minimum amount of interference when acaller crosses from one zone to the next.

As the caller begins leaving a particular cell zone the strength of thesignal from the radio base station begins to diminish. As the signalcontinues to diminish communication becomes difficult, then impossiblewith existing equipment. Another related difficulty is the degradationof signal quality for reasons other than leaving a particular cell zone.For example, interference can cause noise on the channel, makingcommunication difficult. It would be desirable to minimize problems witha noisy or weak signal strength channel being used in a mobilecommunication system. Furthermore, it would be desirable to be able tominimize the above problems with present aircraft radio units having avariety of different traffic service channel usage scenarios.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides an improved method andsystem for performing call handoff.

More specifically, one embodiment the present invention employs adigital system providing call handoff capabilities that greatly improveoverall system call capacity and quality in three different handoffscenarios. In a conservation handoff scenario the present inventionmakes more efficient use of communication channels by combining calls onpartially used traffic service channels to create both more fullyutilized and unused traffic service channels. The resulting unusedchannels are released so that they can be acquired by other aircraft, orby the same aircraft if need be.

An example of a successful conservation handoff would begin with twotraffic service channels being established between an aircraft radiounit on a plane and a radio base station on the ground. At some pointonly one user service channel is in use on each of the two trafficservice channels. Note that each user service channel supports one call.A conservation handoff has the effect of transferring one of the callsfrom its user service channel on one of the traffic service channels tothe unused user service channel on the other traffic service channel.The system identifies when certain criteria fully described below havebeen satisfied. If the criteria have been satisfied a duplicate userchannel of one of the traffic service channels is established on theother traffic service channel. Control is transferred to the trafficservice channel with the duplicate user service channel. The originaluser service channel is broken down and that traffic service channel isreleased. Thus after a conservation handoff has been performed with twopartially used traffic service channels, there remains only one fullyutilized traffic service channel with two active user service channelsand one unused traffic service channel that is released, thus freeingthe unused traffic service channel for use by another aircraft. Theeffect is a traffic packing which maximizes the efficient use of trafficservice channels by reducing their number to the lowest numberpractically possible, thus enabling new call traffic to be carried onvia the released traffic service channels. A conservation handoff isperformed between an aircraft and the same radio base station ordifferent radio base stations and is effectively imperceptible by theuser.

In a Seizure handoff scenario, call handoff improves the signal qualityof calls by effectively handing a traffic service channel off from thecurrent radio base station to a new radio base station with bettersignal attributes. Seizure handoff occurs when one traffic servicechannel is not in use and another is having difficulty communicatingclearly. Typically, a Seizure handoff is used to transfer traffic as anaircraft is flying out of range from the current radio base station intothe range of the new radio base station. Another cause for handoff wouldbe interference caused by external stimuli. In either case a degradationin call quality would be detected, as is fully described below, and ahandoff would be performed to escape the interfering source. ASeizure-type call handoff (Seizure handoff) is performed inside anaircraft radio unit when 1 or 2 user service channels are in use on afirst traffic service channel and a second traffic service channel isidle. The idle traffic service channel seizes a channel at the new radiobase station. Once the channel is established, the user service channelsare transferred from the old traffic service channel to the new trafficservice channel. Once the transfer is complete, the old traffic servicechannel is broken down and the calls are now being carried by the newtraffic service channel on different radio base station depending on theavailable traffic service channels. A Seizure handoff is transparent tothe user as only a few pulse code modulation (PCM) frames are lostduring the transfer.

Another advantage with Seizure handoff is that the selection of the newradio base station also facilitates traffic grooming by choosing a radiobase station with less traffic than other candidate ground stations. Anaircraft can typically see 4 to 6 radio base stations (ground stations)at cruising altitude. By selecting less congested ground stations,traffic within that group of 4 to 6 ground stations is spread evenly,thus reducing the possibility of all the traffic going through one radiobase station. Traffic grooming also benefits aircraft that can't “see”(communicate with) as many ground stations by attempting to maintainfree channels at all ground stations within a quadrant.

Similar to the Seizure handoff scenario, call handoff is utilized in aReservation handoff scenario to improve the signal quality of calls.However, in a Reservation Handoff scenario the airborne radio unit hasboth of its traffic service channels in use when conditions such asdeteriorating signal quality indicate that a handoff is desirable forthe calls operating on one or both traffic service channels. Note thatthe call handoff criteria are evaluated on a per traffic service channelbasis. For a call handoff in a Reservation Handoff scenario (Reservationhandoff), the new traffic service channels are selected and one or bothof the traffic service channels are keyed down. Then the aircraft radiounit aircraft resets its operating frequency and is keyed up on a newchannel that was reserved specifically for the handoff. Synchronizationis re-achieved, and the voice/data path (call) is reconnected. Duringthe handoff period in one embodiment of the present invention, the userexperiences less than 2 seconds of silence while the handoff is inprocess. The same benefits in signal quality improvement and trafficgrooming are derived as in Seizure handoff.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to thedrawings in which:

FIG. 1 is a diagram of the electromagnetic broadcast frequency spectrumallotted to communications utilize by one embodiment of a call handoffsystem of the present invention;

FIG. 2 is an overview block diagram of one embodiment of the presentinvention;

FIG. 3 is a block diagram of one embodiment of the present invention;

FIGS. 4A-4E are block representations of a conservation call handoff.

FIGS. 5A-5E are block representations of a seizure call handoff.

FIGS. 6A-6E are block representations of a reservation call handoff.

FIGS. 7-15 is a flowchart representing the steps taken in a conservationcall handoff.

FIGS. 16-20 is a flowchart representing the steps taken in a seizurecall handoff.

FIGS. 21-251 is a flowchart representing the steps taken in areservation call handoff.

FIGS. 26A-26C are a representation of equipment passing signals overtime to perform a conservation call handoff.

FIGS. 27A-27C are a representation of equipment passing signals overtime to perform a conservation call handoff.

FIG. 28 is a representation of equipment passing signals over time toperform a conservation call handoff.

DESCRIPTION OF THE PREFERRED EMBODIMENT

While the invention is susceptible to various modifications andalternative forms, a specific embodiment thereof has been shown by wayof example in the drawings and will be described in detail. It should beunderstood, however, that it is not intended to limit the invention tothe particular form described, but, on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

Limitation of other present systems have been overcome in the presentsystem that provides means for conducting radio frequency telephonecommunications between many airline passengers and telephones served bylandline telephone networks in a managed system.

Referring now to the drawings, and particularly to FIG. 2, one preferredembodiment of the invention is described. The invention is related to animproved air/ground digital communications system 30 that operates tointerconnect telephones (not shown here) contained aboard manygeographically spaced operational aircraft 32 with a public switchedtelephone network 34. The system 30 also includes one or moregeographically regionalized radio base stations 36 connected to a groundswitch node 38. Each radio base station 36 selectively transmits andreceives radio frequency signals 40 with the aircraft, whilesimultaneously relaying communications with the ground switch node 38,which ultimately switches the traffic to the public switched telephonenetwork (PSTN) 34. In the case of a single radio base station 36, thiscommunication is conducted via one pilot signal 20 and a plurality ofradio frequency traffic service channels 22. Each radio base station 36broadcasts a single pilot signal 20 for the benefit of all aircraft 32within communication range of that radio base station 36. This pilotsignal 20 informs aircraft 32 of traffic service channel 22 availabilityand frequency, and helps aircraft 32 to select a radio base station 36for optimal, long term reception. Each radio base station 36 alsoutilizes user service channels 42 within the traffic service channels 22to transmit and receive radio frequency encoded conversations 40 ofpassengers on aircraft 32.

The system 30 of the present invention manages a finite frequencyspectrum 10. example, a 2 MHz frequency spectrum for each ofbase-to-aircraft and aircraft-to-base communication links is dividedinto 290, 6 KHz frequency channels. The present invention contemplatesthe inclusion of any number of transceivers within the radio basestation 36, up to the predetermined number of user service channels 42available within the finite frequency spectrum 10. However, as apractical matter radio base stations 36 located in areas which tend toexperience lower levels of communication traffic may include fewertransceivers than the radio base stations 36 that reside in highercommunication traffic areas. Preferably, no single radio base station 36will include a transceiver for each user service channel 42 in theentire available frequency spectrun 10 because of a near certainprobability that a large number of such transceivers would never becalled into use.

Referring now to FIG. 3, the specific features of the digitalcommunications system 30 are explained. A cabin telecommunications unit44 having several integrated functions is installed aboard each aircraft32 as part of an aircraft phone system 46. The cabin telecommunicationsunit 44 provides an interface to the user/customer in the form of one ormore telephones distributed throughout the aircraft 32 cabin. The cabintelecommunications unit 44 provides a user interface to voice, fax anddata transmission services. Included are the telephone handsets andcabling. Note that U.S. Pat. No. 4,419,766 discloses techniques whichmay be adopted to make comparisons based upon signal strength andDoppler frequency shift error in order to select a radio base station 36in accordance with the present invention. When a user requests placementof a call on the system 30, the cabin telecommunications unit 44requests a traffic service channel 22 and an aircraft radio unit 48selects an available traffic service channel 22 emanating from a radiobase station 36 that the aircraft radio unit 48 most recently found toprovide the best reception. Handoff is performed as fully describedbelow.

The cabin telecommunications unit 44 provides a physical interfacebetween users and the digital aircraft radio unit 48. Cables 50,specifically shielded twisted pair wires, connect the cabintelecommunications unit 44 to the digital aircraft radio unit 48. Theaircraft radio unit 48 is itself coupled to a aircraft antenna unit 52through coaxial cables 54 for sending and receiving radio transmissions.The aircraft radio unit 48 also includes controls for continuallyscanning through all the potential pilot signals 20 in the system 30 todetermine the radio base station 36 of optimal reception as is describedbelow. Note that the aircraft antenna unit 52 is a blade antenna. Theaircraft radio unit 48 transmits and receives radio frequency signals 10in accordance with the allotted frequencies described above for userservice channels 42 and pilot channels 20. Furthermore, the aircraftradio unit 48 contains two separate transmitters and two receivers tosupport two traffic service channels 22 for voice, data, TDD (hearingimpaired) and fax transmissions.

The aircraft phone system 46 can contain a plurality oftransmitter/receivers (transceivers) to communicate over a plurality oftraffic service channels 22. In one embodiment the aircraft phone system46 communicates over two traffic service channels 22 with each trafficservice channel 22 have two user service channels 42. Thus presently,the maximum number of calls which may simultaneously take place throughany single aircraft 32 is limited to 2 times the number of transceiversincluded in the aircraft 32. As a practical matter the number oftransceivers in an aircraft 32 will be limited to minimize expense andweight and to generally match the potential availability of user servicechannels 42 within the overall communication system 30 of the presentinvention.

The aircraft radio unit 48 contains transceivers, each connected by acoaxial cable to a signal combiner/splitter to combine outgoingcommunications and separate incoming communications from outgoingcommunications. The signal combiner/duplexer is connected to the antennaunit 52 by coaxial cable 54.

The interactions of the aircraft system will now be described. The cabintelecommunications unit 44 serves to direct operation of the aircraftphone system 46 of which it is a part. The cabin telecommunications unit44 controls the aircraft phone system 46 with an aircraft processor (notshown) located in the aircraft radio unit 48, which has access andcontrol to the transceivers. Thus, in response to commands from theaircraft processor, the transceivers transmit and receive signals to andfrom one or more radio base stations 36 over user service channels 42.

A digital code is incorporated into each aircraft transmission to alinked radio base station 36 in order to uniquely identify thetransmitting aircraft 32. The aircraft processor in the aircraft radiounit 48 controls the transfer of call communications from the aircrafttransceivers in the aircraft radio unit 48 to the cabintelecommunications unit 44 which provides the customers with theircalls. Both visual and audio interface can be provided.

User service channel calls 42 are transmitted and received by a commonantenna (not shown) in the aircraft antenna unit 52 and relayed to areceiver front end (not shown), which is a broad band receiver stagecapable of receiving and amplifying signals covering the entire allottedspectrum 10 in the aircraft radio unit 48. The receiver front endprovides signal gain and provides an intermediate frequency signal thatis sent to all transceivers in the aircraft 32. The signal splitterreduces aggregate call signals into individual call signals. Suchaggregate call signals originate from the radio base stations 36.

Pilot channel 20 signals enable asynchronous communications between theaircraft processor and the radio base station 36. The aircraft radiounit 48 is tuned to the pilot channel assigned to it which itdemodulates to receive a pilot signal channel data stream. Under controlof the aircraft processor the aircraft radio unit 48 monitors all pilotchannels, receives the pilot signal data stream and all information fromwhich signal strength and Doppler frequency shift error are calculated.The aircraft radio unit 48, via the aircraft processor, utilizes thisinformation in selecting the radio base station 36 that will provideoptimal reception. The aircraft processor also designates which trafficservice channels of that radio base station 36 the aircraft is toutilize.

The digital communications system 30 further includes a large number ofradio base stations 36. Each radio base station 36 includes a groundantenna unit 56 and a ground radio unit 58. The ground radio unit 58further includes a ground radio baseband processor (not shown) thatlocally executes commands and controls the ground radio unit 58. Also,the radio baseband processor performs local housekeeping tasks for itsradio base station 36 and controls communications with the ground switchnode 38. The ground antenna unit 48 includes an antenna connected via acoaxial cable to a preamplifier (not shown). The preamplifier amplifiessignals received by the antenna in the ground antenna unit 48 and passesthe signals via another coaxial cable (not shown) to a downconverter(not shown) within the ground radio unit 58. The down converter convertsthe composite signal at 894-896 MHz to a 70 MHz (nominally intermediatefrequency (IF)). The IF is distributed to transceivers within the groundradio unit 58 using coaxial cables (not shown).

A plurality of radio base stations 36 are provided to simultaneouslytransmit and receive conversations between one or more aircraft 32 and aradio base station 36 network. Control is conducted over a single pilotsignal 20. Communications is conducted over a plurality of user servicechannels 42 (two user service channels 42 in one embodiment) for eachtraffic service channel 22 on each radio base station 36. The pilotsignal 20 is broadcast for the benefit of all aircraft 32 within rangeof a given radio base station 36, i.e., within that cell. The pilotsignal 22 serves to help each aircraft n selecting a radio base station36 for optimal, long term reception, informs aircraft of theavailability and frequency of currently available traffic servicechannels 22 (which carry encoded conversations).

The pilot signal is a continuously transmitted radio signal fortransmissions of a ground-to-air control channel. The aircraft radiounit uses the pilot signal to: detect the presence of an radio basestation 36 (using radio frequency (RF) signal strength and correctreception of broadcast data as a measure), determine radio base station36 characteristics, and determine traffic service channel 22availability (using a free channel list broadcast by the groundstation).

The aircraft radio unit will monitor pilot channel characteristicsincluding received signal quality, signal strength, rate of change ofsignal strength and pilot doppler shift for all radio base stations 36which are RF-visible. Relative rankings will be determined from thesefactors. The radio base station 36 farthest ahead of the aircraft, ofusable signal strength and signal quality and having at least one freetraffic service channel 22, will be ranked highest. If no usable radiobase station 36 exists ahead of the aircraft, the highest ranking radiobase station 36 behind the aircraft will be that which has usable signalquality and the highest signal strength.

The aircraft radio unit 48 will be tuned to the pilot signal it isassigned and will monitor the uplink 12 data stream for messagesdirected to that aircraft 32. Each time conditions described below crossa threshold the pilot monitoring will switch to a new radio base station36. A radio control link is exchanged between the aircraft radio unit 48and the ground radio unit 58 over the traffic service channel 22. Theradio control link carries the control signals passed that enable ahandoff to occur.

Each ground radio 58 is capable of scanning all user service channels 42assigned to an radio base station 36 and acting as a pilot channel 20transmitter.

The radio base station 36 is connected to the ground switch node througha ground baseband processor 62. The ground baseband processor 62 isconnected to the switch control processor 60 which in turn is connectedto a ground telecommunication unit 64. The ground telecommunication unit64 is coupled to the public switched telephone network 34 (PSTN) toallow calls to occur with any customer coupled to the PSTN 34. Morespecifically, the ground baseband processor 62 is the equipmentresponsible for translating between PCM (pulse code modulation) voicetraffic from the aircraft radio unit 48 and low bit rate voice trafficfrom the PSTN 34.

Each traffic service channel 22 typically contains two user servicechannels 22. The number of user service channels 22 in use iscoordinated between the aircraft radio unit 48 and the ground basebandprocessor. Each user service channel 42 is capable of transporting bothcall-related information (traffic, control, and credit card information)and non-call-related information (alarm, maintenance, and managementinformation).

Each ground radio unit 58 will scan the traffic service channel 22carrier frequencies assigned to its radio base station 36 and determine,per FCC Memorandum of Opinion and Order, whether each traffic servicechannel 22 carrier frequency is in use. The ground radio unit 58 willthen construct a free channel list containing all free traffic servicechannel 22 carrier frequencies to which ground radios 58 have beentuned. 18 The free channel list will be transmitted over the pilotchannel 20 as part of the radio base station 36 information broadcastwithin the ground to air control pilot channel 20. The ground radio unit58 will also generate and transmit a list of all traffic service channel22 carrier frequencies in use. The aircraft radio unit uses the freechannel list to select the highest ranking radio base station 36 wheninitiating a call.

Digital signals are transmitted from the aircraft antenna radio unit 52are detected by receiving antennas in the ground antenna unit 56 on theground, preamplified, reduced to component channels by adownconverter/receive splitter (not shown), and distributed to theplurality of radio base station 36 transceivers. Similarly, outgoingdigital signals from the plurality of radio base station 36 transceiversundergo aggregation in a transmit combiner, before being broadcast by atransmitting antenna.

The ground baseband processor 62 controls the interface between theground switch node 38 and the radio base station 36, it also performslocal housekeeping tasks.

Additionally, when regional aircraft occupy previously vacant voicechannels, the radio base station 36 responds by adjusting the pilotsignal 20 to reflect the newly occupied user service channels 42.

Aboard each aircraft 32, an aircraft radio unit 48 is coupled with theaircraft antenna unit 52 to sample all pilot signals offered within thereception area of the aircraft. The air radio controller compares therelative strength and Doppler frequency shift error of each of the pilotsignals and selects the radio base station 36 that will supply thestrongest, most enduring service.

For transmitting, each ground radio unit 58 is capable of transmittingthe pilot signal with the ground antenna unit 56, however, only one isdesignated to do so by the radio baseband processor. The remainder ofthe ground radios are allocated to carry calls (voice and data traffic)on traffic service channels 22 selected by the ground radio basebandprocessor.

The output of each ground radio is an 849-851 MHz signal that containspilot channel 20 data and user voice and data traffic. The ground radiounit's transmitter is connected via coaxial cable to acombiner/amplifier (not shown). The combiner/amplifier, combines the lowlevel output of the ground radio unit 58 and amplifies the compositesignal to 10 watts/channel. The composite transmit signal is thenfiltered using a band pass filter whose center frequency is 850 MHz(nominally). The output of the transmit bandpass filter is then passedvia coaxial cable to a transmit antenna in the ground antenna unit 56.

Each radio base station 36 transceiver contains a microprocessor basedI/O interface (not shown) which is used to communicate with equipmentexternal to the transceiver, such as the radio baseband processor. Eachradio base station 36 is designed to accommodate common types of groundradios within the ground radio unit 58. However, ground radiospreferably have common I/O designs, so that control from the radiobaseband processor can be uniformly administered.

Accordingly, the ground radio baseband processor sends a command to eachof transceivers to specify which frequency channel, if any, to use inconducting communications between radio base station 36 and aircraft 32.Preferably, to prevent interference, no two transceivers utilize acommon frequency channel. Commands may be sent from the radio basebandprocessor to transceivers within the ground radio unit 58 in real-timeto dynamically reallocate frequency channels.

In response to the radio baseband processor, a primary broadcasttransmitter sends the pilot signal 20 of the radio base station 36 toany aircraft 32 within range. Broadcast transmitters are arranged tooperate in a configuration, so that the radio baseband processor oralarm system can promptly cause a switch to substitute anothertransceiver for the primary broadcast transmitter in case the latterfails. Such a failure condition might arise, for example, when the powerlevel of the primary broadcast transmitter falls below a predeterminedthreshold.

The radio baseband processor performs the central computing functionsfor each radio base station 36, including regulation of operation of theradio base station 36 components, directing radio base station 36communications, and coordinating use of the radio base station 36'sassigned spectrum with other services. In the preferred embodiment, theradio baseband processor includes a computer based upon an Intel 80386microprocessor designed to operate at 20 MHz.

As discussed above, each radio base station 36, through its primary orsecondary broadcast transmitter, broadcasts a pilot channel 20 datastream for receipt by all aircraft systems within range of the radiobase station 36 (RBS). Each of these streams of data represents amessage which is continually repeated and updated as necessary.Generally speaking, the message includes channel availabilityinformation which aircraft systems use in deciding which voice channelsto communicate upon.

Turning now to FIGS. 4, 5 and 6 there is illustrated generally how callhandoff is performed in three scenarios, i.e., conservation handoff,seizure handoff and reservation handoff. Each type of handoff isexplained in much greater detail below. Note that the types of usercalls carried on each user service channel is for illustrative purposesonly. Each user service channel has the capability of carrying all typesof communications defined above.

In FIGS. 4A-4E a conservation handoff is generally shown. Beginning inFIG. 4A, an initial situation with a first aircraft radio unit trafficservice channel (ARAD TSC 1) has both user service channels,specifically, user service channel 1 (USC1) and user service channel 3(USC3) carrying a voice call (VOICE 1) and a facsimile call (FAX 1),respectively. Aircraft radio unit traffic service channel 2 (ARAD TSC 2)is carrying on subband B user service channel 5 (JSC5) and user servicechannel 7 (USC7), carrying data call 1 (DATAl) and voice call 2 (VOICE2), respectively. FIG. 4A illustrates two fully utilized traffic servicechannels. FIG. 4B shows FAX1 and VOICE2 as discontinued, leaving twopartially used traffic service channels (a conservation-type callhandoff situation). Conservation handoff begins in FIG. 4C where theDATA1 call is duplicated on USC3, the “new” user service channel, whilecontinuing on USC5, the “old” user service channel. In FIG. 4D the DATA1call is effectively transferred to USC3 and the old user service channel(USC5) is idled. Next, in FIG. 4E the aircraft radio unit trafficservice channel 2 (ARAD TSC 2) is turned off, allowing that trafficservice channel to be acquired by a new caller on a different aircraftor the same aircraft.

In FIGS. 5A-5E a seizure handoff is generally shown. Beginning in FIG.5A, an initial situation with a first aircraft radio unit trafficservice channel (ARAD TSC 1) has both user service channels in use,specifically, user service channel 1 (USC1) and user service channel 3(USC3) carrying a first voice call (VOICE 1) and a second voice call(VOICE 2), respectively. Aircraft radio unit traffic service channel 2(ARAD TSC 2) is off. FIG. 5A illustrates one fully utilized trafficservice channel and one idle traffic service channel when communicationsquality begin to deteriorate (a seizure situation). Seizure handoffbegins in FIG. 5B which shows ARAD TSC 2 locating an appropriate subbandand channel on the same or another radio base station. In FIG. 5C bothcalls on ARAD TSC 1, specifically VOICE 1 and VOICE 2 are duplicated on“new” user service channels USC5 and USC7, respectively. In FIG. 5D, the“old” user service channels USC1 and USC3 are idled, while VOICE 1 andVOICE 2 calls continue on the “new” user service channels USC5 and USC7.Next, in FIG. 5E the aircraft radio unit traffic service channel 1 (ARADTSC 1) is turned off, allowing that traffic service channel to beacquired by a new caller on a different aircraft or the same aircraft.

In FIGS. 6A-6E a reservation handoff is generally shown. Beginning inFIG. 6A, an initial situation with a first aircraft radio unit trafficservice channel (ARAD TSC 1) has both user service channels,specifically, user service channel 1 (USC1) and user service channel 3(USC3) carrying a first voice call (VOICE 1) and a second voice call(VOICE 2), respectively. Aircraft radio unit traffic service channel 2(ARAD TSC 2) also has both user service channels in use, specifically,user service channel 5 (USC5) is carrying a third voice call (VOICE 3)and user service channel 7 (USC7) is carrying a data call (DATA). FIG.6A illustrates two fully utilized traffic service channels when, like inthe seizure handoff scenario, communications quality begin todeteriorate on ARAD TSC 1 (a reservation situation). Note that theprimary distinction between a seizure scenario and a reservationscenario is whether there is an idle traffic service channel for theexisting calls to be switched to. In FIG. 6B, unlike in FIG. 5B, no userservice channels have been idled, however, a search is made for the besttraffic service channels to switch VOICE 1 and VOICE 2 to. Actualhandoff begins in FIG. 6C which shows ARAD TSC 1 keying down VOICE 1 andVOICE 2, leaving USCI and USC3 idle. In FIG. 6D, ARAD TSC 1 has locateda new subband and channel. In FIG. 6E, VOICE 1 and VOICE 2 calls arecontinued on the “new” user service channels USC1 and USC3, nowoperating on a different channel.

Turning now to FIGS. 7-28 there is illustrated specifically how callhandoff is performed in three scenarios, i.e., conservation handoff,seizure handoff and reservation handoff.

A flow chart illustrating conservation call handoff is shown in FIGS.7-15. In FIG.7 the start block 100 defines the initial condition of thesystem just prior to the present invention being ready to perform aconservation call handoff. A conservation handoff will not be performedunless a conservation situation to known to exist. In the presentinvention a conservation situation is defined as a situation in whichthere are at least two traffic service channels 22 wherein each trafficservice channel 22 has only one respective user service channel 42 inuse by customer calls. If other factors permit, as is more thoroughlydescribed below, the calls on the traffic service channels 22 arecombined into one traffic service channel 22 and the other trafficservice channel 22 can potentially be released.

In step 102 the present invention determines whether a conservationsituation exists by checking the status of every traffic service channel22. If a conservation situation does not exist, then control of thepresent invention continues waiting for the condition to occur. If thereis, then control of the present invention proceeds to step 104. In step104 control checks to see if there are any other calls waiting in aqueue and any user service channels 42 are available. If so, then thosecalls are processed in step 106 then control returns to step 102. Ifthere are no other calls waiting in step 104, then control proceeds tostep 108, wherein a timer is initiated. In step 110, control looks atwhether the timer TSC_CONS_HO is equal to 30. If not control proceeds tostep 112 where the timer is incremented by 1 every second. In step 114control examines whether there is no longer a conservation situation,specifically, whether both traffic service channels are still partiallyin use, if not go to step 100, if so go back to step 110. In step 110 ifthe TSC_CONS_HO timer equals 30 seconds go to step 116 in FIG. 8. Inshort, if the conservation situation ends before the timer counts thirtyseconds, i.e. there is no longer at least two user service channels 22in use during the thirty seconds, then control returns to step 100,otherwise, control goes to step 116.

In step 116 on FIG. 8, control determines whether either of the twopartially used traffic service channels 22 is communicating with anisolated cell. There are two types of an isolated cells, one is a radiobase station 36 which is not attached to a ground switching node that ispart of the GTE Airfone network, the other type of isolated cell is notoverlapped with other cells within the range of the aircraft 32. Ifeither of the two traffic service channels 22 are communicating with anisolated cell then that traffic service channel 22 is marked as beingconnected to an isolated cell and is accordingly treated as beingunavailable for call handoff and control stops the handoff in step 118,if not, control proceeds to step 120. In step 120, control determineswhether any of the user service channels 22 are being switched betweenvoice and data transmissions, if so control will wait until the switchis complete. If no switch is occurring or has completed, controlproceeds to step 122, where control determines whether the digitalaircraft radio unit 48 is in an administrative state. If the digitalaircraft radio 48 is locked, thus allowing a maintenance action to beconducted, then control will wait until the administrative state isunlocked. If the digital aircraft radio 48 is not in an lockedadministrative state, or no longer in an locked administrative state,then control proceeds to step 124 to determine if another type ofhandoff is already in progress. If another type of handoff is inprogress, control waits for that handoff to complete before continuing.Note that control continuously checks to see if the conservation handoffsituation exists. If at any time the conditions for handoff are notsatisfied then the handoff is canceled, even if the handoff is beingheld in abeyance. If there is no longer any other type of handoff inprogress, control proceeds to step 126 where the conservation handoffprocess is begun.

In FIG. 9, a conservation handoff is begun in step 130 where the digitalaircraft radio 48 sends a message (INIT_CONS_HO) to the cabintelecommunication unit 44. The message contains information identifyingwhich user service channels 42 and traffic service channel 22 of thecall to be moved and its destination user service channels 42 andtraffic service channel 22. The cabin telecommunication unit 44 respondsto the INIT_CONS_HO message in step 132 by sending the call's voice ordata (assume voice and data is all inclusive) on both the present userservice channels 42 and the destination user service channels 42 inparallel. Once the INIT_CONS_HO message is received, call requests forthe present and destination user service channels 22 are queued untilthe handoff is complete. Furthermore, the cabin telecommunication unit44 sends an acknowledgement signal (INIT_CONS_HO_ACK) in step 134 to thedigital aircraft radio 48 indicating that parallel transmission istaking place. Control proceeds to step 136 wherein the digital aircraftradio 48 creates a message containing information about the old and newchannels, handoff type, in this case a conservation handoff, theaircraft ID, a call reference number, a call bearer capability, i.e.voice, data, fax, or TDD (telephone device for the deaf), and thedestination traffic service channel 22 and user service channels 42.

Next the aircraft radio unit 48 sends the HO_SEIZ_REQ message via thenew user service channel to the radio base station 36. Detail of thisprocess is provided in part “A” 140 of the flow chart beginning in FIG.10.

On FIG. 10, part A 140, step 142, the aircraft radio unit 48 sends adata ready gold code to the radio baseband processor in the radio basestation 36 which in turn forwards the message to a ground basebandprocessor 62 in the ground switching node 38. The message is sent on achannel that will carry the new user service channel. If data receivedby the ground baseband processor 62 has an incorrect gold code, thusindicating error, then the ground baseband processor 62 will continue toreceive the aircraft radio unit 48 IDLI transmissions until the goldcode is followed in step 144. The gold code is a known way of orderingbits and check bits to allow errors in transmission to be detected. Ifthe gold code is received and indicates no errors are present then theground baseband processor 62 sends an acknowledge signal (ACK1) to theaircraft radio unit 48 in step 146, acknowledging readiness to receivedata, otherwise the radio base station 36 will continue sending the IDL1gold code to the aircraft radio unit 48 in step 148. The radio basebandprocessor in the ground radio unit 58 forwards the message to itsaircraft radio unit 48. in step 150, if the aircraft radio unit 48 didnot receive the acknowledge signal (ACK1) from the ground basebandprocessor before a time out of 2 seconds the aircraft radio unit 48 willbegin seizure request again by incrementing the retry counter in step152. Then in step 154 if the retry counter is equal to three (it beganat zero before step 152) then handoff is stopped in step 118, otherwise,sequence “A” 140 is begun again at 140.

However, if the acknowledge signal (ACK1) was received then the aircraftradio unit 48 sends the following informational elements: a protocoldiscriminator, message type indicator, PCM channel, handoff type,aircraft ID call reference, call bearer capability and destinationtraffic service channel/user service channel ID to the ground basebandprocessor as part of a (TX_INBAND_DATA) message at step 156. In steps158-176 of a gold code retry block, control uses a counter to retrysending the specified gold code three times before giving up in step118. If a CRC (cyclic redundancy check—an error detection code)calculated from the message received from the aircraft radio unit iscorrect when compared against a valid CRC transmitted with the abovemessage in step 158, then in step 160 the ground baseband processor 62sends an IDL2 gold code to the aircraft radio unit 48 acknowledgingreceipt of the valid message, then control proceeds to step 162.

In step 158, if the CRC received is not valid then control proceeds tostep 164 where the ground baseband processor 62 sends a data acknowledgenot gold code (DACKN gold code) to the aircraft radio unit 48 andcontrol proceeds to step 162. In step 162, if the aircraft radio unit 48received DACKN from the ground baseband processor 62 then the aircraftradio unit 48 increments a retry counter in step 166 and control flowsto step 168. If the retry counter is equal to 3 in step 168 then handoffis stopped in step 118, if not then in step 170 the aircraft radio unitreceives DACKN and sends the data message again as it did in step 156and control flows to step 158. Back in step 162, if IDL2 gold code wasreceived by the aircraft radio unit 48 then control proceeds to step172. In step 172 the aircraft radio unit 48 determines if all the datafor the seizure request has been sent by examining whether this is thelast data message transmission for this message. If it is not thencontrol flows back to step “B” 156, if it is then in step 174 theaircraft radio unit 48 sends IDL2 gold code to the ground basebandprocessor 62 when the data message is complete in step 174. Next, instep 176 the ground baseband processor 62 sends a DACK (dataacknowledge) gold code to the aircraft radio unit 48 when the groundbaseband processor 62 determines that the entire message has beentransmitted and receives and IDL2 gold code from the aircraft radio unit48. Note that the system retries all gold code signalling as describedabove in steps 158-176 if error is encountered.

Returning to FIG. 9, in step 178 the new ground baseband processor 62forwards the data message to the switch control processor 60. In step180 the switch control processor 60 sends a handoff request message to aground telecommunications unit 64 within the ground switching node 38specifying a new logical port inside the ground telecommunication unit64 with the associated call reference number. More specifically, insideof the ground telecommunications unit 64 there is a configuration tablethat associates physical and logical ports carrying calls. The logicalport number of the old and new ports on the ground telecommunicationsunit 64 is used by the switch control processor 60. The call referencenumber is used to avoid confusion between calls from the same aircraftby allocating a unique number with every user call. The call referencenumber is assigned sequentially by the cabin telecommunications unit 44starting with the number 1 every time the aircraft phone system 46powers up. In step 182 the ground telecommunications unit 64 sends userservice channels 42 voice/data to the old and new ground basebandprocessors 62. More specifically, upon receipt of the handoff requestmessage, parallel user service channels 42 call paths are established onold and new ground baseband processors, any state changes to the call,or process of new calls on proposed handoff channels are held inabeyance.

In FIGS. 9 and 12, step 184 is labelled “C” to indicate it providesdetail for the three steps 178, 180 and 182. In FIG. 12, step 186 theaircraft radio unit 48 sends an acknowledge signal (ACK2) in a gold codeformat to the ground baseband processor 62 over the new user servicechannels 42 in order to acknowledge receipt of the ground basebandprocessor's 62 data acknowledge signal (DACK) on the new user servicechannels 42. As part of step 186, in part 188 the aircraft radio unit 48sets a digital signal processor to a new bearer capability. Bearercapability refers to the different software routines used for digitallysignal processing voice, data, facsimile and TDD calls. Once the propersoftware routine is loaded, the radio is considered “capable” ofcarrying the various types of call traffic. In step 190, if the groundbaseband processor 62 did not receive the ACK2 gold code then control isreturned to step 184, if it was received then control proceeds to step192.

In step 192 the ground baseband processor 62 sets its digital signalprocessor (not shown) to new bearer capability, i.e. voice, data, fax,TDD, based on the handoff seizure request message. In step 194 theground baseband processor 62 sends the aircraft radio unit 48 an IDL3gold code. More specifically, after the bearer capability is set the newground baseband processor sends over the new user service channels 42the IDL3 gold code indicating the bearer capability is set. In step 196if the aircraft radio unit 48 received the IDL3 gold code controlproceeds to step 198 in FIG. 13, if not the ground baseband processorretries the IDL3 gold code transmission in step 200 as described abovewith regard to the gold code retry block described above. If theaircraft radio unit received the IDL3 gold code then in step 198, FIG.13, the aircraft radio unit 48 sends the new ground baseband processor62 the IDL3 gold code upon receipt of IDL3 gold code from the new groundbaseband processor 62 on the new user service channels 42, indicatingreadiness for user data.

Steps 202 and 204 indicate where the ground baseband processor 62 waitsto receive the IDL3 gold code from the aircraft radio unit 48 with anappropriate gold code retry in step 204. In step 206, when the groundbaseband processor 62 receives the IDL3 gold code from the aircraftradio unit 48 it sends back to the aircraft radio unit 48 an answer goldcode (ANS) followed by user data on the new user service channels 42. Instep 208 if the aircraft radio unit 48 received the ANS signal from thenew ground baseband processor 62 then in step 209 the aircraft radiounit 48 sends an acknowledge signal (ACK3) to the new ground basebandprocessor 62 on the new user service channels 42. However, if the ANSsignal was not received by the aircraft radio unit 48 then the aircraftradio unit 48 initiates in steps 210-214 a 60 second counter duringwhich time the signal must be received or the handoff attempt will beaborted. In step 216 if the ACK3 signal was received by the new groundbaseband processor 62 then control is returned to step 218 on FIG. 9.

In FIG. 9, step 218, the ground telecommunications unit 64 sends ahandoff request response to the switch control processor 60 both are inthe ground switching node 38) acknowledging that the resources requestedare available and that the parallel user service channels on the old andnew ground baseband processors 62 have been established.

In FIG. 14, step 220 the switch control processor 60 sends to the radiobaseband processor the handoff seizure response message to the radiobaseband processor that will carry the new user service channel. In step222 the switch control processor 60 sends a handoff seizure responsemessage (HO_SEIZ_RESP) to the aircraft radio unit 48 using the radiocontrol link. The message informs the aircraft radio unit 48 that theground path for the new user service channel is established. In step 224the aircraft radio unit 48 sends an acknowledgment signal (ACK)indicating that the ground path for the new user service channels 42 isestablished. In step 226 the aircraft radio unit 48 sends a handoffcomplete signal (HO_COMP) to the cabin telecommunications unit 44instructing the cabin telecommunications unit 44 to switch both thetransmit and receive paths to the new user service channel. In step 228the cabin telecommunications unit 44 connects the new user servicechannel and disconnects the old channel. In step 230 the cabintelecommunications unit 44 sends a handoff complete acknowledge(HO_COMP_ACK) to the aircraft radio unit 48 when the cabintelecommunications unit 44 has completed switching the call to the newuser service channel and disconnecting the call from the old userservice channel. In step 232 the aircraft radio unit 48 sends IDL1N andNCR (normal call release) gold codes to the ground baseband processor 62on old user service channel upon receipt of the handoff completeacknowledge signal from the cabin telecommunications unit 44 . Theaircraft radio unit 48 sends IDL1N and normal call release (NCR) goldcodes to the radio baseband processor on the old user service channel.The radio baseband processor forwards the codes to the old groundbaseband processor 62. In step 234 the old ground baseband processor 62sends a handoff release (HO_RLS) signal to the switch control processor60. In step 236 the ground baseband processor 62 sends IDL1N and NCRgold codes to the aircraft radio unit 48 acknowledging receipt of thehandoff call release and forwarding call release to the switch controlprocessor 60.

In FIG. 15, step 238, the switch control processor 60 sends the handoffrelease signal (HO_RLS) to the ground telecommunications unit 64. Instep 240 the ground telecommunications unit 64 releases the old audiopath from the old ground baseband processor 62. In step 242 the groundtelecommunications unit 64 sends a handoff release response signal(HO_RLS_RESP) to the switch control processor 60. Once the oldvoice/data path is disconnected from the ground baseband processor 62the ground telecommunications unit 64 send a handoff release signal tothe switch control processor 60 indicating the completion of aconservation handoff. Steps 244-252 describe the resumption of normalcall service following a conservation handoff. Although the new trafficservice channel may be on the same radio base station 36 with the sameground baseband processor 62 it may also be on a new radio base station36. Thus, for the sake of clarity the ground baseband processor 62carrying the old traffic service channel will be called the old groundbaseband processor 62 and the ground baseband processor 62 carrying thenew traffic service channel will be called the new ground basebandprocessor 62, even though in some cases they are the same.

Call handoff in a seizure-type scenario (seizure handoff), asillustrated in FIGS. 16-20 switches voice/data calls from an old trafficservice channel(the one currently in use) to a new traffic servicechannel (the destination). One can assume the protocol and handshakebetween different components is the same as under a conservation handoffwhere similar actions are performed.

In FIG. 16, steps 300-340 illustrate the various transmission signalcharacteristics examined by one embodiment of the present invention. Insteps 302-310 the aircraft radio unit 48 monitors the aircraft radiounit 48 received signal strength to determine whether it has fallenbelow a threshold of −107 dBm (decibels above a milliwatt) for 15seconds. In steps 312-320 the aircraft radio unit 48 monitors theaircraft radio unit 48 bit error rate to determine if the rate has risenabove 0.5% for 15 seconds. In steps 322-330 the aircraft radio unit 48monitors the ground radio 58 receive signal strength to determine if ithas fallen below a threshold of −110 dBm for a 30 second period. Insteps 332-340 the aircraft radio unit 48 monitors the ground radiobaseband processor 62 to determine if the bit error rate has risen above1%. If any of the above thresholds are exceeded for the durationspecified, than a seizure handoff scenario exists and a seizure handoffis initiated in step 342 of FIG. 17.

In FIG. 17, step 342, if the traffic service channel is communicatingwith an isolated cell then seizure call handoff is terminated in step344, if not then proceed to step 346. In step 346 if a voice/dataswitchover, i.e. the user switching between voice and data, is alreadyin progress then the system waits for completion, otherwise proceed tostep 348. In step 348 if the aircraft radio unit 48 is in a lockedadministrative state then control will wait until that state isunlocked, otherwise proceed to step 350. In step 350 if another handoffis already in progress then again the system will wait until thathandoff is completed, otherwise proceed to FIG. 18 step 352.

In FIG. 18, step 352, the aircraft radio unit 48 seizes the new trafficservice channel and a corresponding new user service channels at a newradio base station 36 using aircraft phone system 46 hardware notpresently in use. In step 354 the aircraft radio unit 48 sends aninitiate seizure handoff signal (INIT_SEIZE_HO) to the cabintelecommunications unit 44. In step 356 the cabin telecommunicationsunit 44 sends voice/data from the call or calls on both the old and newuser service channels. In step 358 the cabin telecommunications unit 44sends an acknowledgement signal (INIT_SEIZE_HO_ACK) to the aircraftradio unit 48 acknowledging parallel voice paths have been establishedon the old and new user service channels. In step 360 the aircraft radiounit 48 sends a handoff seizure request signal (HO_SEIZURE_REQ) to thenew ground baseband processor 62 on one or both user service channelsvia the new traffic service channel. Detail on step 360 is the same asin the conservation handoff in section “A”. In step 362 the new groundbaseband processor 62 forwards the HO_SEIZURE_REQ signal to the switchcontrol processor 60. Detail on step 362 is the same as in theconservation handoff in section “C”.

In FIG. 19, step 364, the switch control processor 60 sends the handoffrequest signal (HO_REQ) to the ground telecommunications unit 64. Instep 366 the ground telecommunications unit 64 sends the voice/data tothe old and new ground baseband processors 62 for the user servicechannels with user traffic. In step 368 the ground telecommunicationsunit 64 sends the handoff request response (HO_REQ_RESP) to the switchcontrol processor 60 once parallel voice paths have been established. Instep 370 the switch control processor 60 sends the new radio basebandprocessor the HO_SEIZE_RESP message. In step 372 the radio basebandprocessor sends the HO_SEIZE_RESP message to the aircraft radio unit 48via the radio control link of the new traffic service channel 22. Instep 374, the aircraft radio unit 48 acknowledges the message to the newradio baseband processor via the radio control link.

In FIG. 20, step 376, the aircraft radio unit 48 sends a handoffcomplete signal (HO_COMP) to the cabin telecommunications unit 44. Instep 378 the cabin telecommunications unit 44 connects the new userservice channel(s) to the callers and disconnects the old user servicechannel(s). In step 380 the cabin telecommunications unit 44 sends thehandoff complete acknowledgement signal (HO_COMP_ACK) to the aircraftradio unit 48. In step 382 the aircraft radio unit 48 sends a normalcall release signal to the old ground baseband processor 62 on the oldtraffic service channel. In step 384 the old ground baseband processor62 forwards the normal call release to the switch control processor 60to release the old traffic service channel. In step 386 the switchcontrol processor 60 sends the normal call release message to the groundtelecommunications unit 64. Step 388 occurs in parallel with step 390.In step 388 the ground telecommunications unit 64 disconnects oldvoice/data path for one or both user service channels on the old trafficservice channel. In step 390 the old ground baseband processor 62 send anormal call release to the aircraft radio unit 48 on the old trafficservice channel. In step 392 the aircraft radio unit 48 releases the oldtraffic service channel for use by other aircraft 32. Finally, in step394 the ground telecommunications unit 64 sends a handoff releaseresponse signal (HO_RLS_RESP) to the switch control processor 60 oncethe old voice/data path(s) are disconnected. Handoff is complete andback to step 344.

Call handoff in a seizure-type scenario is similar to call handoff in areservation type scenario (reservation handoff). The key distinction isthat there are no free traffic service channels 22 available on theaircraft radio unit 52 to switch to. Thus a reservation handoff mustclose both old (currently being used) traffic service channels 22 andbring up the voice/data calls on two new (switched to) traffic servicechannels 22 in one embodiment of the present invention. Reservationhandoff is illustrated in FIGS. 21-26. One can assume the protocol andhandshake between different components is the same as under aconservation handoff, seizure handoff and a reservation handoff wheresimilar actions are performed. Thus, the “old” and “new” designationsare used as described above with regard to seizure handoff.

In FIG. 21, steps 400-440 illustrate the various transmission signalcharacteristics examined by one embodiment of the present invention in acall reservation scenario. In steps 402-410 the aircraft radio unit 48monitors the aircraft radio unit 48 received signal strength todetermine whether it has fallen below a threshold of −107 dBm for 15seconds. In steps 412-420 the aircraft radio unit 48 monitors theaircraft radio unit 48 bit error rate to determine if the rate has risenabove 0.5 % for 15 seconds. In steps 422-430 the aircraft radio unit 48monitors the ground radio baseband processor 62 receive signal strengthto determine if it has fallen below a threshold of −110 dBm for a 30second monitoring period. In steps 432-440 the aircraft radio unit 48monitors the ground radio baseband processor 62 to determine if the biterror rate has risen above 1%. If any of the above thresholds areexceeded for the duration specified, and in step 442 there are no idletraffic service channels 22, then a reservation handoff scenario existsand a reservation handoff is initiated in step 444 of FIG. 22, otherwisea seizure handoff scenario exists.

In FIG. 22, steps 446-452 symbolically represent the increased amount oftime the thresholds of steps 402, 412, 422, 432, respectively, must beexceeded for the system to initiate a reservation handoff. The amount oftime is increased because service to the existing calls in a reservationhandoff is interrupted, albeit for less than 2 seconds. If the trafficservice channel is communicating with an isolated cell in step 454, thenreservation call handoff is terminated in step 456, if not then proceedto step 458. In step 458 if a voice/data switchover, i.e. user switchingbetween voice and data, is already in progress then the system waits forcompletion, otherwise proceed to step 460. In step 460 if the aircraftradio unit 48 is a locked administrative state then control will waituntil that state is unlocked, otherwise proceed to step 462. In step 462if another handoff is already in progress then again the system willwait until that handoff is completed, otherwise proceed to FIG. 23, step464.

In FIG. 23, step 464, the aircraft radio unit 48 transmits a reservationhandoff request (HO_RES_REQ) to the old radio baseband processor via theradio control link of the traffic service channel to be handed off. Instep 466 the old radio base station 36 sends the HO_RES_REQ signal tothe switch control processor 60 via the ground radio baseband processor62. In step 468 the switch control processor 60 forwards the HO_RES_REQsignal to the new ground radio baseband processor via the pilot channel20. In step 470 the new ground radio baseband processor reserves therequested traffic service channel for the reservation handoff or assignsa traffic service channel if the requested traffic service channel isnot available. The aircraft radio unit 48 will try to select the mostoptimal traffic service channel it determines is available based on thesignal characteristics examined above. In step 472 the new radiobaseband processor sends the reservation handoff response message(HO_RES_RESP) to the switch control processor 60. In step 474 the newradio baseband processor removes the reserved traffic service channelfrom the free channel list.

In FIG. 24, step 476, the switch control processor 60 sends the handoffrequest signal (HO_REQ) to the ground telecommunications unit 64. Instep 478 the switch control processor 60 sends a reservation handoffsetup message (HO_RES_SETUP) to the new ground baseband processor 62. Instep 480 the ground telecommunications unit 64 sends the handoff requestresponse signal (HO_REQ_RESP) to the switch control processor 60 toindicate that the new traffic service channel and associated userservice channels are ready for calls. In step 482 the switch controlprocessor 60 sends the old radio baseband processor the HO_REQ_RESPsignal. In step 484 the old radio baseband processor sends the handoffrequest response signal (HO_REQ_RESP) to the aircraft radio unit 48 viathe radio control link of the old traffic service channel. In step 486the aircraft radio unit 48 send an acknowledge message to the old radiobaseband processor via the radio control link of the old traffic servicechannel.

In FIG. 25, step 488, the aircraft radio unit 48 sends a normal callrelease on the old traffic service channel to be handed off. In step 490the old ground baseband processor 62 sends a normal call release to theswitch control processor 60 to release the old traffic service channel.In step 492 the ground baseband processor 62 sends a normal call releaseto the aircraft radio unit 48 on the old traffic service channel. Instep 494 the switch control processor 60 sends the handoff release(HO_RLS) signal to the ground telecommunications unit 64. In step 496the ground telecommunications unit 64 connected the existing calls'voice/data/etc. to the new ground baseband processor and sends thehandoff release response signal (HO_RLS_RESP) to the switch controlprocessor 60. In step 498 the aircraft radio unit 48 releases the oldtraffic service channel, turns the transmitter off and tunes to the newtraffic service channel. In step 500 the aircraft radio unit 48 seizesthe reserved traffic service channel at the new radio base station 36.Next, in step 502, the aircraft radio unit 48 sends the IDL1N gold codeto the new ground baseband processor on the new traffic service channelto indicate service recovery for the handed off calls. In step 504 thenew ground baseband processor sends IDLlN gold code to the aircraftradio unit 48, thereby acknowledging service recovery. In step 506 theaircraft radio unit 48 and the new ground baseband processor switch toappropriate voice/data mode for each user service channel on the newtraffic service channels 22. The handoff then stops in step 456. Thusthe reservation handoff process is completed and the system returns tochecking for the poor signal characteristics specified above.

In FIGS. 26-28, all three types of call handoffs are illustrate from asignalling perspective between the various functional units of thecommunications system 30. Specifically, conservation handoff isillustrated in FIG. 26, seizure handoff is illustrated in FIG. 27, andreservation handoff is illustrated in FIG. 28.

Thus, there has been described herein an improved air/ground digitalcommunications system 30.

Many modifications and variations of the invention as hereinbefore setforth can be made without departing from the spirit and scope thereofand therefore only such limitations should be imposed as are indicatedby the appended claims.

What is claimed is:
 1. A method of performing a call handoff of at least one user's call carried on a first traffic service channel, said user being located in an airplane, said user's call being transmitted between an aircraft phone system and a radio base station, said radio base station coupled to a public switched telephone network, comprising the steps of: identifying, with said aircraft phone system, when a second traffic service channel's unused call capacity is not less than a capacity to carry said at least one user's call; duplicating, with said aircraft phone system in response to the step of identifying, said at least one user's call onto said second traffic service channel to create a duplicate call over said second traffic service channel; transferring, with said aircraft phone system, the at least one user's call from said first traffic service channel to said second traffic service channel using the duplicated call; and releasing, with said aircraft phone system, said first traffic service channel. 